This invention relates primarily to resistive structures for television cathode ray tubes for performing such functions as: (1) protecting a tube and its associated circuitry from destructive electrical arcs and arc currents, and from voltages and currents induced by such arcs; (2) eliminating static charge accumulations on insulating surfaces inside a tube; (3) providing a mechanism within a tube for voltage division; (4) providing internal resistive-capacitive signal coupling networks.
The envelope of a television cathode ray tube comprises a glass funnel and a mating faceplate. The funnel has a neck within which is located an electron gun. The faceplate of the tube has a fluorescent screen on which is impressed a very high DC voltage -- typically in the range of 20-30 kilovolts or more.
Conductive coatings on the inside and outside of the funnel serve as a capacitor which filters the high voltage supplied to the screen. The inner conductive coating is at screen potential and also serves to transmit the screen voltage to the neck of the tube where it is applied to a high voltage anode electrode at the forward end of the electron gun.
The electron gun has for each electron beam a cathode and a series of closely spaced electrodes which shape, accelerate and focus the electron beam(s) generated in the gun. To accomplish these functions, the various electrodes require widely different electrical potentials. The large voltage difference established between certain high voltage and low voltage electrodes in the gun creates a susceptibility to arcing between the electrodes, e.g. should there exist particulate foreign matter in the inter-electrode space, a burr on an electrode, a misaligned or improperly spaced electrode, or the like. When the conditions for arcing are right, the high voltage filter capacitor, with its immense stored electrical energy, will in a few microseconds dump its stored charge, inducing transient currents in the associated receiver circuitry.
Because the instantaneous peak arc currents can reach hundreds of amperes in magnitude, great destruction can be wrought by such arcs. External circuitry can be damaged; internal gun parts can be eroded to the point of inoperability or severely reduced in their effectiveness. High arc currents are capable of sputtering electrode materials onto adjacent surfaces, resulting in the formation of electrical leakage paths. Further, arcing in a tube during its normal operation can result in a loud audible report which may be quite disturbing to a viewer.
In recent years, the design evolution of color picture tubes has taken a direction tending to exacerbate the arcing problem. The desire for greater picture brightness has driven the screen voltage inexorably upward toward and even beyond 30 kilovolts. A trend toward wider beam deflection angles and a desire to minimize power consumption have dictated the use of tubes with smaller neck diameters. A small neck diameter implies a more closely confined environment for the electron gun, with the attendant increased probability of arcing between components of the electron gun assembly or between the gun assembly and the containing tube envelope.
In order to reduce tube arcing, it is routine today to design color tubes and electron gun assemblies with every effort to maximize intercomponent spacing, to minimize points of field concentration, and otherwise to configure the tube and gun structures to minimize the tendency of a tube to arc. After a tube receives its electron gun and the envelope is sealed, it is commonplace to "spot-knock" (high voltage condition) the gun. "Spot-knocking" is an operation wherein a pattern of fluctuating and constant voltages of high magnitude are applied to the tube to "knock" (remove) loose particles which may have lodged between gun electrodes, burrs on electrode parts, and other agents which might lead to arcing of the tube during its normal operation. Typically peak voltages during spot knocking are much higher than the screen operating voltage. Spark gaps, diodes, filters, gas discharge lamps, and other protective devices are commonly provided in the associated receiver (at significant cost) to protect receiver circuitry from damage by arc induced currents and voltages.
Television picture tube manufacturers have long attempted to develop an internal resistive element which would be coupled in series with the high voltage filter capacitor and the electron gun to suppress the magnitude of arc currents and thereby to overcome the potentially destructive effects of arcing in the tube during tube operation.
The requirements for such an internal resistance element are, however, extremely severe. Following are some of the requirements, not necessarily in their order of importance, of an internal resistive element for protecting a television picture tube against arcing.
Requirement 1. The resistive element must be compatible with the clean high vacuum environment inside a cathode ray tube. The element cannot emit gas which would significantly decrease the tube's vacuum level or impair the performance of the cathode in the electron gun assembly. The element cannot flake, erode, ablate, or otherwise generate particles which might block openings in a color selection electrode or lodge in a gap between gun electrodes.
Requirement 2. The resistive element must be compatible with the tube's fabrication processes. Perhaps the most severe of the fabrication processes are the high temperature cycles which a color tube is subjected to when the faceplate is sealed to the funnel and during exhausting (and sealing) of the tube. Temperatures may reach 430.degree. C. or higher during these high temperature operations.
Requirement 3. The resistive element must have an effective impedance which is relatively stable at a selected point in an appropriate impedance range -- for example a few kilohms to tens of megohms. If the dynamic impedance of the element is too low, inadequate protection against arc currents will be provided. If the DC resistance of the element is too high (e.g. 10.sup.12 ohms), the material will act as an insulator and collect stray charges which may alter the electron beam paths or initiate arcing. Further, if the DC resistance of the element is too high, the voltage drop across the element as a result of leakage current flowing through it will result in an intolerable drop in the voltage applied to the anode electrode of the gun. A resistive element may have an appropriate DC resistance but, when situated in a finished tube, have a stray capacitance which is so high as to establish a low dynamic impedance path for an arc transient.
Whereas it has long been known that an internal arc suppression resistor must have an appropriate value of DC resistance, it is believed that the distributed (stray) capacitance problem has not been fully appreciated.
Requirement 4. The resistive element cannot be physically obtrusive to the electron beams. As noted, there is a very limited amount of space available in the neck of a television cathode ray tube, particularly a tube of the small neck type, and particularly in the region near the front of the electron gun. Because of this space limitation, it has proven to be difficult to design a non-obtrusive discrete internal resistive element.
Requirement 5. The resistive element must be capable of being satisfactorily electrically terminated at each end. If the resistive element is a neck coating, it has been found that even modest arc currents are apt to cause localized heating of the glass underneath the contact point(s) with the result that the glass may chip or become predisposed toward eventual failure. It is difficult to maintain contact integrity after a number of arcs have occurred.
Requirement 6. The television industry being highly competitive, the resistive element and its cost of installation must be low enough to be commercially viable.
Requirement 7. The requirement last to be described is not last in importance -- it is perhaps the reason that no television tube having within it an arc suppressing resistive element has been marketed commercially. This seventh requirement is that the resistive element not be susceptible to being by-passed or nullified due to conductive deposits on the surface thereof.
Specifically, all television cathode ray tubes today utilize a "flashed" (vaporized) "getter" material which "gets" (adsorbs) residual gases in the tube after the tube has been pumped down as far as is practicable and sealed off. The gas-adsorptive getter material most commonly employed is a barium compound. Barium is highly electrically conductive, however. When the getter is flashed, a conductive barium coating is deposited on substantially all exposed areas within the tube. In order to "get" the greatest quantity of residual gas, the getter must be flashed over a wide area inside the tube; inevitably, getter material is deposited on the resistive element.
It is clear that any resistive element used for arc suppression, voltage division or the like, will be effectively by-passed or nullified if a shunt path around the resistive element is created by conductive getter material. Attempts to reduce the area of getter flash by shielding or by use of directional evaporation of getters reduce the pumping speed and effectiveness of the getter. As noted, this seventh requirement is perhaps the thorniest of all and has stalled development of commercially practicable internal arc suppression resistive systems.